1. Field of the Invention
The present invention relates to an air intake device for a multi-cylinder internal combustion engine.
2. Background Art
Japanese Patent JP-A-2005-325696 discloses an air intake device of a V-type engine. The air intake device includes a surge tank having an internal space divided into an upper portion and a lower portion. The surge tank is disposed upwardly of the engine. The upper portion of the internal space of the surge tank functions as a surge tank for a first bank of the engine. The lower portion of the internal space of the surge tank functions as a surge tank for a second bank of the engine. A partition between the upper portion and the lower portion of the surge tank includes an on/off valve. With the on/off valve closed, the upper portion of the surge tank is shut off from the lower portion thereof. With the on/off valve open, the upper portion of the surge tank communicates with the lower portion thereof.
Generally speaking, a small surge tank volume improves torque in low to middle speed regions and response, while reducing torque in a high speed region. A large surge tank volume, on the other hand, improves torque in the high speed region and reduces torque in the low to middle speed regions.
In the air intake device of the known art, the upper portion and the lower portion of the surge tank each function as a small surge tank for the corresponding bank with the on/off valve closed in the low to middle speed regions. In the high speed region, the upper portion and the lower portion of the surge tank function integrally as a large surge tank with the on/off valve opened. This improves engine torque throughout the entire speed region.
Including the above-mentioned document, the applicant is aware of the following documents as a related art of the present invention.    [Patent Document 1] Japanese Patent JP-A-2005-325696    [Patent Document 2] Japanese Patent JP-A-2007-16755    [Patent Document 3] Japanese Patent JP-A-2003-307159
The air intake device of the known art, however, has the following problem. Specifically, when the on/off valve is opened, a vane of the on/off valve advances into the upper and lower tank space to block an air flow, leading to increased suction resistance.
In addition, in the air intake device of the known art, it is necessary to make the on/off valve as large as possible in order to have a sufficiently large opening area when the on/off valve is open. A large on/off valve, however, contributes to a narrow flow path cross-sectional area when the on/off valve is open, which makes it necessary to make the surge tank higher. Such an air intake device as this one requires that the surge tank be disposed in a space between the engine and an engine hood, imposing stringent restrictions on the height of the surge tank. If the surge tank is tall, therefore, it makes it impossible to dispose the surge tank at an optimum position, at which each cylinder has an equal intake pipe length. This results in a problem of varying intake pipe lengths among different cylinders.